1. Field of the Invention
The present invention generally relates to image forming devices, and more specifically to an image forming device which executes control of the first stage transfer process of an intermediate transfer body to improve the image quality of copying machines, printers, facsimile devices, and multifunction devices which employ electrophotographic methods.
2. Description of the Related Art
Conventionally, image forming devices employ electrophotographic methods to form toner images one of each color consecutively with corresponding image support bodies. Upon the intermediate transfer body, which moves in a treadmill motion, the color images are superposed one onto the other and the first stage transfer image formed upon a intermediate transfer body is transferred to the target transfer body or recording medium (transfer material, paper). More specifically, the transfer process of an intermediate transfer unit is comprised of a first stage transfer process which develops the electrostatic latent image with toner on the image support body and transfers the toner image to the intermediate transfer body, and a second stage transfer process which transfers the toner image transferred on to the intermediate transfer body to the recording medium; and after these transfer processes a fixation process bonds the toner image to the recording medium.
Due to there being two processes, the first stage transfer process and the second stage transfer process, the electrically charged toner image is susceptible to damage by mechanical or electrostatic means. Specifically in the first stage transfer, when passing each station, a discharge occurs in the minute air gap between the intermediate transfer body and the image support body, and due to the discharge the electrical charge quantity distribution of the toner held by the intermediate transfer body broadens. Toner that becomes charged up, neutralized, or of the same polarity is realized and if the second stage transfer to the recording medium is executed with the existence of such toner, toner which cannot be electrostatically transferred to the recording medium remains upon the intermediate transfer body, and an image deficiency or a so called “bosotsuki” image occurs. Specifically, when using a recording medium with large surface roughness such as discharge-prone recycled paper the electrostatic attraction of toner is difficult, and due to pressure distribution being spotty the mechanical binding of toner with a small electrical charge quantity is difficult as well.
To prevent “bosotsuki” images occurring, for example, Japanese Laid-Open Patent Application 2000-293055 discloses detecting the recording medium that is to be used in the second stage transfer of the toner image and depending upon the type of the recording medium switching the strength of the first stage transfer bias of the first stage transfer unit.
To increase productivity, when executing successive printing or both-sides printing, the intervening spaces between images are sometimes narrowed to a size smaller than those of the spaces between the image support bodies of a four tandem structure. Also when applying a first stage transfer bias, as means to hold down electricity consumption and achieve desired downsizing, the integration of the power sources of the four color image support bodies which power sources apply the same bias is known.
Assume the structure of the device disclosed in Japanese Laid-Open Patent Application 2000-293055, which presuppose the use of a revolving structure, is applied to the four tandem structure and successive multiple color images are printed. The start timing of the first stage transfer of the N+1 page recording medium is at a timing while the first stage transfer of the N page recording medium is in progress. If there were a mixture of various types of recording media so that the paper quality changes, in a machine type which simultaneously applies bias to multiple first stage transfer units, electrical noise occurs if the first stage transfer bias value of the N page recording medium is changed to the first stage transfer bias value of the N+1 page recording medium (in a case where the toner image straddles stations before or after the change to the bias) and dissatisfaction with the image quality results.